# APC-Mutated MUC4-Positive Fibroblastoma With Cytoplasmic β-Catenin: A Novel Variant Expanding Its Immunophenotypic Spectrum

## TL;DR

A new variant of MUC4-positive fibroblastoma with APC mutations is identified, showing different β-catenin behavior than previously known.

## Contribution

A novel APC-mutated MUC4-positive fibroblastoma variant with cytoplasmic β-catenin absence is described.

## Key findings

- The tumor showed diffuse cytoplasmic MUC4 but lacked cytoplasmic β-catenin staining.
- Biallelic APC inactivation was confirmed molecularly in the case.
- This expands the immunophenotypic spectrum of MUC4-positive fibroblastoma.

## Abstract

MUC4-positive fibroblastoma is a newly characterized fibroblastic neoplasm, typically presenting as a bland spindle cell tumor with diffuse cytoplasmic MUC4 expression, nuclear β‑catenin positivity, and APC gene alterations. We report a novel case in a 34-year-old woman demonstrating diffuse cytoplasmic MUC4 expression but negative cytoplasmic β‑catenin staining, with molecular confirmation of biallelic APC inactivation. This finding broadens the known immunophenotypic spectrum of MUC4-positive fibroblastoma and challenges current assumptions regarding β‑catenin localization in APC-altered soft tissue tumors. Additional clinical, radiologic, and long-term follow-up data may help refine our understanding of the biological behavior and diagnostic boundaries of this emerging entity.